Stretching over the top of a square white basin, Nathan Butler, 10, and Thomas Baek, 9, were arms deep in water as horseshoe crabs scurried around and a diamondback terrapin poked its head above water.

It’s not the first time the fourth graders from Pointers Elementary School in Clarksville, Maryland, have seen horseshoe crabs, but it is the first time they’ve held them. They’ve been raising several of them in a class during recess at school.

“It was different than like holding an animal like a turtle. They pinch a bit, but it doesn’t hurt,” Butler said. “It feels like they are tickling you.”

The kids were among the hundreds at the DuPont Nature Center at Mispillion Reserve Saturday for the fifth annual Peace, Love and Horseshoe Crab festival.

The festival celebrates the shelled arthropod that is currently populating the shores of the Delaware Bay to spawn. They have a unique relationship with the local ecosystem, humans and shorebirds said Dawn Webb, a trainer and educator with the state division of Fish and Wildlife.

“They help people and animals, and in turn teach people how to help them,” Webb said.

Horseshoe crabs’ blood helps humans through pharmaceuticals, and their protein-rich eggs help feed the migrating shorebirds that attract so many to Delaware’s shores. People help the crabs by flipping them off their backs to save them, Webb noted.

“This time of the year when they come onto the shorelines to spawn, they can get stranded,” Webb said. “Humans can help them by turning them over. It’s rescuing them.”

Under a hard exterior shell are a set of fine gills the crabs need to breathe. If they are flipped over or fall on their backs with the gills exposed to the sun, they can dry out and die, Webb said.

It’s up to humans to flip them back over, and the crabs pay it forward. Their blood, rich in iron, is used to test vaccinations before human use.

“Anything that would mix with your bodily fluid is tested using horseshoe crab blood,” Webb said.

Their eggs keep migratory shorebirds nourished and on track during their long treks.

Along the deck of the nature center were a group of experienced bird watchers, gazing through powerful scopes in hopes of catching a glimpse of the red knot, a medium-sized shorebird in the midst of its annual migration.

The bird ends up around the Delaware Bay this time of year coming up from South America. When it arrives, its typically emaciated, said Kevin Kalasz, a biodiversity program manager at DNREC and manager of the Delaware shorebird project.

The birds, which have a distinct red rust belly area, feast on horseshoe crab eggs, leaving the Delaware Bay plump and ready for the trip ahead, Kalasz said.

Birders Marica Brake and her husband, Hilton, had never been to the festival before.

“It’s beautiful,” Brake said. “We definitely wanted to be up here when the horseshoes were up. And of course, the red knots.”

Contact Jon Offredo at (302) 678-4271 or at joffredo@delawareonline.com or on Twitter @jonoffredo.